Fin Spacer Protected Source and Drain Regions in FinFETs

ABSTRACT

A method includes forming Shallow Trench Isolation (STI) regions in a semiconductor substrate and a semiconductor strip between the STI regions. The method also include replacing a top portion of the semiconductor strip with a first semiconductor layer and a second semiconductor layer over the first semiconductor layer. The first semiconductor layer has a first germanium percentage higher than a second germanium percentage of the second semiconductor layer. The method also includes recessing the STI regions to form semiconductor fins, forming a gate stack over a middle portion of the semiconductor fin, and forming gate spacers on sidewalls of the gate stack. The method further includes forming fin spacers on sidewalls of an end portion of the semiconductor fin, recessing the end portion of the semiconductor fin, and growing an epitaxial region over the end portion of the semiconductor fin.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 14/056,649, filed Oct. 17, 2013, and titled “Fin Spacer Protected Source and Drain Regions in FinFETs,” which application is incorporated herein by reference.

This application is related to U.S. patent application Ser. No. 13/740,373, filed Jan. 14, 2013, and entitled “Semiconductor Device and Fabricating the Same,” Attorney Docket No. 2012-0701/24061.2264; U.S. patent application Ser. No. 13/902,322, filed May 24, 2013, and entitled “Semiconductor Device and Method of Fabricating Same,” Attorney Docket No. TSM13-0232/24061.2471; U.S. patent application Ser. No. 13/934,992, filed Jul. 3, 2013, entitled “Fin Structure of Semiconductor Device,” Attorney Docket No. TSM13-0414US; U.S. patent application Ser. No. 13/970,295, filed Aug. 19, 2013, entitled “FinFETs and Methods for Forming the Same,” Attorney Docket No. TSM13-0463US; U.S. patent application Ser. No. 13/952,753, filed Jul. 29, 2013; U.S. patent application Ser. No. 14/024,148, filed Sep. 11, 2013, entitled “Isolation Structure of Fin Field Effect Transistor,” Attorney Docket No. TSM13-0800US; and U.S. patent application Ser. No. 14/090,072, filed Nov. 26, 2013, and Attorney Docket No. TSMC2013-1042.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. For example, a three dimensional transistor, such as a fin-like field-effect transistor (FinFET), has been introduced to replace a planar transistor. Although existing FinFET devices and methods of fabricating the FinFET devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. Improvements in this area are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 through 11 are cross-sectional views and perspective views of intermediate stages in the manufacturing of a Fin Field-Effect Transistor (FinFET) in accordance with some exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

A Fin Field-Effect Transistor (FinFET) and the method of forming the same are provided in accordance with various exemplary embodiments. The intermediate stages of forming the FinFET are illustrated. The variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIGS. 1 through 11 are perspective views and cross-sectional views of intermediate stages in the manufacturing of a FinFET in accordance with some exemplary embodiments. FIG. 1 illustrates a perspective view of an initial structure. The initial structure includes wafer 100, which includes substrate 20. Substrate 20 may be a semiconductor substrate, which may further be a silicon substrate. Substrate 20 may be doped with a p-type or an n-type impurity. Isolation regions such as Shallow Trench Isolation (STI) regions 22 are formed to extend from a top surface of substrate 20 into substrate 20, wherein top surface of substrate 20 is also a major surface 100A of wafer 100. The portions of substrate 20 between neighboring STI regions 22 are referred to as semiconductor strips 21. The top surfaces of STI regions 22 may be leveled with the top surfaces of hard mask layers 25, which are used as a Chemical Mechanical Polish (CMP) stop layer during the formation of STI regions 22. Pad oxide layers 23 are formed between hard mask layers 25 and semiconductor strips 21.

FIG. 2 illustrates the replacement of the top portion of semiconductor strips 21 in FIG. 1 with semiconductor strips 24. In the replacement of semiconductor strips 21, pad oxide layers 23 and hard mask layers 25 (FIG. 1) are removed first. At least the top portions, or substantially entireties, of semiconductor strips 21 are then removed to form recesses (not shown, occupied by semiconductor strips 24). Next, an epitaxy is performed to epitaxially grow semiconductor strips 24 in the recesses, forming the structure in FIG. 2.

The formation of semiconductor strips 24 includes depositing a semiconductor layer 24A to partially fill in the trenches, and a semiconductor layer 24B on the top of first semiconductor material 24A. The first and second semiconductor layers 24A and 24B may be deposited by epitaxial growing processes. The epitaxial processes include Chemical Vapor Deposition (CVD) techniques (e.g., Vapor-Phase Epitaxy (VPE) and/or Ultra-High Vacuum CVD (UHV-CVD)), molecular beam epitaxy, and/or other suitable processes. In some embodiments, semiconductor layer 24A is a silicon germanium (SiGe) layer, and semiconductor layer 24B is a silicon layer free from or substantially free from germanium (for example, with a germanium percentage lower than about 5 percent). Semiconductor layer 24A may have a germanium percentage in the range between about 15 percent and about 60 percent, although the germanium percentage may be higher or lower. The thickness of semiconductor layer 24A may be in the range between about 10 nm and about 40 nm.

After the formation of the structure as shown in FIG. 2, a CMP process is performed to remove excessive semiconductor layer 24B, and to planarize the top surfaces of semiconductor layer 24B and STI regions 22.

Referring to FIG. 3, STI regions 22 are recessed, so that the top portions of semiconductor strips 24 are higher than the top surfaces of STI regions 22 to form semiconductor fins 26. In accordance with some embodiments, interface 28 between semiconductor layer 24B the respective underlying semiconductor layer 24A are higher than top surface 22A′ of the recessed STI regions 22. In alternative embodiments, interfaces 28 are level with or lower than top surfaces 22A′. Dummy oxide layer 30 may be formed on the sidewalls and the top surfaces of semiconductor fins 26. Dummy oxide layer 30 may be formed, for example, through deposition.

Referring to FIG. 4, gate stack 32 is formed on the top surface and the sidewalls of semiconductor fins 26. In some embodiments, gate stack 32 includes dummy gate electrode 34 over dummy dielectric 30. Dummy gate electrode 34 may be formed, for example, using polysilicon. In some embodiments, height (thickness) H1 of dummy gate electrode 34 is in the range between about 80 nm and about 120 nm. Gate stack 32 may also comprise hard mask layer 36 over dummy gate electrode 34. Hard mask layer 36 may include silicon nitride and/or silicon oxide, for example, and may be a single layer or a composite layer including a plurality of layers. In some embodiments, hard mask layer 36 includes silicon nitride layer 36A and silicon oxide layer 36B over silicon nitride layer 36A. Height H2 of silicon nitride layer 36A may be in the range between about 10 nm and about 30 nm, and height H3 of silicon oxide layer 36B may be in the range between about 80 nm and about 120 nm. Gate stack 32 may cross over a plurality of semiconductor fins 26 and STI regions 22. Gate stack 32 has a lengthwise direction substantially perpendicular to the lengthwise direction of semiconductor fins 26. In accordance with the embodiments of the present disclosure, gate stack 32 is a dummy gate stack, and will be replaced by a replacement gate in a subsequent step.

FIGS. 5 and 6 illustrate the formation of gate spacers 40 and 42. Referring to FIG. 5, spacer layer 38 is formed as a blanket layer to cover the structure shown in FIG. 4. FIG. 5 illustrates a cross-sectional view taken along a plane, which is the same plane containing line A-A in FIG. 4. In some embodiments, spacer layer 38 is formed of silicon nitride, and may have a single-layer structure. In alternative embodiments, spacer layer 38 is a composite layer including a plurality of layers. For example, spacer layer 38 may include a silicon oxide layer, and a silicon nitride layer over the silicon oxide layer. Spacer layer 38 may be formed as a substantially conformal layer, and hence thickness T1 of the vertical portions of spacer layer 38 on the sidewalls of semiconductor fin 26 and gate stack 32 is close to thickness T2 of the horizontal portion of spacer layer 38. For example, thickness T1 and T2 may have a difference smaller than about 20 percent of thickness T2.

Next, spacer layer 38 is patterned, forming gate spacers 40 and fin spacers 42, as shown in FIG. 6. In some embodiments in which spacer layer 38 (FIG. 5) includes silicon nitride, the patterning of the silicon nitride layer includes a dry etching using CH₂F₂ as an etchant. In other embodiments in which spacer layer 38 (FIG. 5) includes a silicon oxide layer and a silicon nitride layer, the patterning of spacer layer 38 includes a dry etching using CH₂F₂ as an etchant to pattern silicon nitride, followed by a dry etching using CF₄ as an etchant to pattern the silicon oxide layer. The patterning includes an anisotropic effect, so that the horizontal portions of spacer layer 38 are removed, while the vertical portions on the sidewalls of gate stack 32 remain to form gate spacers 40. The vertical portions of spacer layer 38 on the sidewalls of semiconductor fin 26 remain to form fin spacers 42.

The process conditions for patterning gate spacer layer 38 are controlled, so that the top ends 44 of fin spacers 42 are lower than the top surfaces of semiconductor fins 26. Furthermore, the top ends 44 are higher than the top surface of semiconductor layer 24A, which may be a silicon germanium layer. Accordingly, the sidewalls of the entire semiconductor layer 24A are protected by fin spacers 42 and STI regions 22. The resulting fin spacers 42 between two neighboring fins 26 may have a dishing-shape in the cross-sectional view, with the edge portions of fin spacers 42 being highest (with height H4), and the center portion of fin spacers 42 in the middle of neighboring fins 26 being the lowest (with height H5). In some embodiments, the center height H5 of fin spacers 42 is in the range between about 5 nm and about 15 nm. Edge height H4 may be greater than center height H5 by a difference in the range between about 2 nm and about 8 nm.

Referring to FIG. 7, the exposed portions of dummy oxide layer 30, if any, are removed through etching. An etching step is then performed to etch the portions of semiconductor strips 24 (FIG. 3) that are not covered by gate stack 32 and gate spacers 40. When the etching stops, semiconductor layers 24B, which may be silicon layers, still have the bottom portion left, with the thickness of the remaining semiconductor layers 24B being in the range between about 2 nm and about 8 nm, for example. Accordingly, semiconductor layers 24A are not exposed. In some embodiments, the top ends 44 of fin spacers 42 are higher than the top surfaces of the remaining semiconductor layers 24B, hence recesses 46 are formed between neighboring fin spacers 42.

Next, referring to FIG. 8, epitaxy regions 48 are formed by selectively growing a semiconductor material from recesses 46 (FIG. 7) that are left by the removed portions of semiconductor strips 24. In some exemplary embodiments, epitaxy regions 48 comprise silicon phosphorous (SiP) or phosphorous-doped silicon carbide (SiCP). The further epitaxial growth of epitaxy regions 48 causes epitaxy regions 48 to expand horizontally, and facets start to form. As illustrated in FIG. 8, due to the blocking of fin spacers 42, epitaxy regions 48 are first grown vertically in recesses 46, during which time epitaxy regions 48 do not grow horizontally. After recesses 46 are fully filled, the subsequently grown epitaxy regions 48 are grown both vertically and horizontally. Hence, the horizontal growth is delayed.

In some embodiments, during the growth of epitaxy regions 48, an n-type impurity such as phosphorous is in-situ doped to form source and drain regions, which are also denoted using reference numeral 48. The resulting FinFET is hence an n-type FinFET.

Hard mask layer 36 (shown in FIG. 8) is removed by photo patterning and dry etching processes, and top surface of dummy gate electrode 34 (FIG. 8) is exposed. Next, as shown in FIG. 9, Inter-Layer Dielectric (ILD) 50 is formed. Although not shown, a buffer oxide layer and a Contact Etch Stop Layer (CESL) may also be formed before the formation of ILD 50. ILD 50 may comprise a Flowable oxide formed using, for example Flowable Chemical Vapor Deposition (FCVD). A CMP may then be performed to level the top surfaces of ILD 50, top surface of dummy gate electrode 34 (FIG. 8), and gate spacers 40 with each other. Each of ILD 50, fin spacers 42, and insulation regions 22 may have distinguishable interfaces with the other ones of ILD 50, fin spacers 42, and insulation regions 22 since they are formed in different process steps, having different densities, and/or comprise different dielectric materials.

Next, dummy gate electrode 34 as shown in FIG. 8 is removed in an etching step, so that recess 52 is formed to extend into ILD 50, as shown in FIG. 9. To illustrate the features behind the front portion of ILD 50, some front portions of ILD 50, semiconductor strips 21, source/drain regions 48, source/drain regions 48, STI regions 22, and the like are not shown in FIGS. 9, 10, and 11, so that the inner features may be illustrated. It is appreciated that the un-illustrated portions of ILD 50, semiconductor strips 21, source/drain regions 48, and STI regions 22 still exist. After the removal of dummy gate stack 32, the middle portions of semiconductor fins 26 are exposed to recess 52. During the removal of dummy gate electrode 34, dummy gate oxide 30 (FIG. 3) is used as an etch stop layer when the top layer is etched. Dummy gate oxide 30 is then removed.

Referring to FIG. 10, a thermal oxidation process is performed on the exposed first and second semiconductor layers 24A and 24B. In some embodiments, the thermal oxidation process is conducted in an oxygen-containing ambient. In other embodiments, the thermal oxidation process is conducted in a combination of a steam ambient and an oxygen-containing ambient. During the thermal oxidation process, outer portions of semiconductor layer 24A (FIG. 7) are converted to semiconductor oxide regions 54, while some inner portion is not converted to oxide. Semiconductor oxide regions 54 may be silicon oxide regions, and include portions on opposite sides of semiconductor region 56. Semiconductor oxide regions 54 may also form a ring encircling semiconductor region 56, with the ring include a portion overlying and a portion underlying semiconductor region 56 in addition to the portions on opposite sides of semiconductor region 56.

During the thermal oxidation process, the semiconductor layer 24A obtains a volume expansion. In some embodiments, the semiconductor layers 24A and 24B and the thermal oxidation process are configured so that the semiconductor layer 24A obtains a volume expansion to achieve a desired degree of channel strain, such as 1 Gpa of tensile strain. It is realized, however, that the ratios of the volume expansion may change in different embodiments.

In some embodiments, the oxidation is performed at a temperature in the range between about 400° C. and 600° C. The oxidation time may range between about 2 minutes and about 4 hours, for example. The oxidation of silicon in silicon germanium is easier than the oxidation of germanium in the same silicon germanium region. Accordingly, the silicon atoms in semiconductor layers 24A are oxidized, and the germanium atoms in semiconductor layers 24A remain substantially un-oxidized. The germanium atoms may diffuse inwardly toward the centers of semiconductor layers 24A to form semiconductor region 56, in which germanium atoms are concentrated. Accordingly, semiconductor region 56 has a germanium percentage higher than that of semiconductor layer 24A (FIG. 9), and may possibly a pure germanium region. The germanium atoms may also diffuse to the interface regions between semiconductor layers 24A and the underlying semiconductor region 21, and the interface between semiconductor layers 24A and the overlying semiconductor layer 24B. During the oxidation, a silicon oxide layer (not shown) is formed on the top surface and the sidewalls of semiconductor layer 24B. The oxide layer may be kept, with the subsequently formed replacement gate formed on the oxide layer. The oxide layer may also be removed before the subsequent formation of the replacement gate, as shown in FIG. 11.

As a result of the oxidation, the width W1 of germanium/silicon germanium region 56 is smaller than width W2 of the underlying portion of semiconductor strip 21 and width W3 of semiconductor layer 24B. In these embodiments, region 56 may form the bottom portion of the resulting semiconductor fins 26.

FIG. 11 illustrates the formation of the replacement gate comprising gate dielectric 58 and gate electrode 60. The intermediate stages in the formation of gate dielectric 58 and gate electrode 60 are not illustrated, and are described briefly below referring to FIG. 11. Gate dielectric 58 is then formed as a blanket layer in recess 52 (FIG. 10) and on the top surfaces and the sidewalls of semiconductor fins 26. In accordance with some embodiments, gate dielectric layer 58 comprises silicon oxide, silicon nitride, or multilayers thereof. In alternative embodiments, gate dielectric layer 58 comprises a high-k dielectric material. In which embodiments, gate dielectric layer 58 may have a k value greater than about 7.0, and may include a metal oxide or a silicate of Hf, Al, Zr, La, and the like. Next, conductive material 60 is formed over gate dielectric layer 58, and fills the remaining recess 52 (FIG. 10). Conductive material 60 may comprise a metal-containing material such as TiN, TaN, TaC, Co, Ru, Al, Cu, W, combinations thereof, or multi-layers thereof. After the filling of conductive material 60, a CMP is performed to remove the excess portions of gate dielectric layer 58 and conductive material 60, which excess portions are over the top surface of ILD 50. The resulting remaining portions of conductive material 60 and gate dielectric layer 58 thus form a replacement gate for the resulting FinFET 62.

The embodiments of the present disclosure have some advantageous features. By forming the fin spacers that prevent silicon germanium layer 24A to be exposed in the step (shown in FIG. 7) of etching semiconductor fins 26, silicon germanium layers 24A are protected from being etched. If silicon germanium layers 24A are not protected, silicon germanium layers 24A will be etched more than the overlying layer 24B. A hole will be formed under semiconductor layer 24B, which hole will be later filled (may be partially) by epitaxy regions 48 (FIG. 8). The filled epitaxy region 48 in the hole causes the subsequently formed source/drain regions and gate electrode to be too close to each other, and hence causes the increase in the gate-to-source/drain leakage or the shorting of gate and source/drain.

In addition, the fin spacers prevent the pre-mature lateral expansion of the epitaxy source/drain regions. If the epitaxy source/drain regions are laterally expanded starting from a very low level, some of the laterally expanded source/drain regions may merge long before other source/drain regions merge with each other. The merged source/drain regions have much higher growth rates than the un-merged source/drain regions, resulting in a non-uniform growth in source/drain regions. Hence, the fin spacers are beneficial for the uniform growth of source/drain regions.

In accordance with some embodiments, a method includes etching a semiconductor substrate to form a first plurality of recesses and filling the first plurality of recesses to form Shallow Trench Isolation (STI) regions. A portion of the semiconductor substrate between the STI regions forms a semiconductor strip, with edges of the semiconductor strip contacting sidewalls of the STI regions. The method also includes replacing a top portion of the semiconductor strip with a first semiconductor layer and a second semiconductor layer. The second semiconductor layer is formed over the first semiconductor layer, and the first semiconductor layer has a first germanium percentage higher than a second germanium percentage of the second semiconductor layer. The method also includes recessing the STI regions. A portion of the semiconductor strip with edges exposed by the recessed STI regions forms a semiconductor fin. The method further includes forming a gate stack over a middle portion of the semiconductor fin, forming gate spacers on sidewalls of the gate stack, forming fin spacers on sidewalls of an end portion of the semiconductor fin, recessing the end portion of the semiconductor fin, and growing an epitaxial region over the end portion of the semiconductor fin that is recessed.

In accordance with other embodiments, a method of forming a Fin Field-Effect Transistor (FinFET) device includes forming a first and a second semiconductor fin over a substrate, the first and second semiconductor fins being parallel to each other, and forming Shallow Trench Isolation (STI) regions surrounding the first and the second semiconductor fins. Top portions of the first and the second semiconductor fins comprise a first epitaxial layer and a second epitaxial layer, with the first epitaxial layer underlying the second epitaxial layer and having a first germanium percentage. The method also includes forming a gate stack over respective middle portions of the first and the second semiconductor fins, and forming a fin spacer between end portions of the first and the second semiconductor fins. The end portions of the first and the second semiconductor fins are disposed on a same side of the gate stack. The fin spacer extends continuously on a first sidewall of the first semiconductor fin, a top surface of the STI regions, and a second sidewall of the second semiconductor fin. An edge of the fin spacer on the first sidewall has a first height larger than a second height of the fin spacer measured at a point midway between the first and the second semiconductor fins. The method further includes removing top portions of the second epitaxial layer from the respective end portions of first and the second semiconductor fins, and epitaxially growing a first source/drain region and a second source/drain region over remaining portions of the second epitaxial layer of the end portions of the first and the second semiconductor fins, respectively.

In accordance with yet other embodiments, a method of forming a Fin Field-Effect Transistor (FinFET) device includes forming a first and a second semiconductor fin over a substrate, with Shallow Trench Isolation (STI) regions surrounding the first and the second semiconductor fins. Top portions of the first and the second semiconductor fins include a germanium-containing first semiconductor layer and a substantially germanium-free second semiconductor layer, with the first semiconductor layer underlying the second semiconductor layer. The method also includes forming a gate stack over middle portions of the first and the second semiconductor fins and forming fin spacers on sidewalls of end portions of the first and the second semiconductor fins. Top ends of the fins spacers extend above a top surface of the first semiconductor layer. The method further includes removing top portions of the second semiconductor layer, and subsequently, epitaxially growing source/drain regions over the end portions of the first and the second semiconductor fins, removing the gate stack to expose the first and the second semiconductor layers in the middle portions of the first and the second semiconductor fins, and oxidizing outer portions of the first semiconductor layer.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A method comprising: etching a semiconductor substrate to form a first plurality of recesses; filling the first plurality of recesses to form Shallow Trench Isolation (STI) regions, wherein a portion of the semiconductor substrate between the STI regions forms a semiconductor strip, wherein edges of the semiconductor strip contact sidewalls of the STI regions; replacing a top portion of the semiconductor strip with a first semiconductor layer and a second semiconductor layer, wherein the second semiconductor layer is formed over the first semiconductor layer, wherein the first semiconductor layer has a first germanium percentage higher than a second germanium percentage of the second semiconductor layer; recessing the STI regions, wherein a portion of the semiconductor strip with edges exposed by the recessed STI regions forms a semiconductor fin; forming a gate stack over a middle portion of the semiconductor fin; forming gate spacers on sidewalls of the gate stack; forming fin spacers on sidewalls of an end portion of the semiconductor fin; recessing the end portion of the semiconductor fin; and growing an epitaxial region over the end portion of the semiconductor fin that is recessed.
 2. The method of claim 1, wherein the replacing the top portion of the semiconductor strip further comprises: removing a top portion of the semiconductor strip to form a second recess; epitaxially growing a first semiconductor layer in the second recess, the first semiconductor layer comprising germanium; and epitaxially growing a second semiconductor layer in the second recess and over the first semiconductor layer, the second semiconductor layer being substantially free of germanium.
 3. The method of claim 1, wherein the forming the gate spacers and the forming the fin spacers comprise: blanket depositing a spacer layer over the gate stack and the semiconductor fin; and patterning the spacer layer to form the gate spacers and the fin spacers.
 4. The method of claim 3, wherein the patterning the spacer layer comprises: anisotropically etching the spacer.
 5. The method of claim 1, wherein the fin spacers have top ends that are below a top surface of the semiconductor fin and above a top surface of the first semiconductor layer.
 6. The method of claim 1, wherein the forming the gate spacers and the forming the fin spacers comprise depositing a silicon nitride layer, and patterning the silicon nitride layer.
 7. The method of claim 1, wherein the step of recessing the end portion of the semiconductor fin includes recessing a top portion of the second semiconductor layer, whereas the first semiconductor layer and a bottom portion of the second semiconductor layer are not recessed.
 8. The method of claim 1, wherein the step of recessing the end portion of the semiconductor fin forms a third recess over a remaining end portion of the semiconductor fin, wherein the growing the epitaxial region comprises growing the epitaxial region vertically first to fill the third recess before growing the epitaxial region horizontally.
 9. The method of claim 1, wherein the growing the epitaxial region comprises in-situ doping an n-type impurity in the epitaxial region.
 10. The method of claim 1, further comprising: forming an Inter-Layer Dielectric (ILD) over the epitaxial region and the fin spacers; removing the gate stack to form a fourth recess in the ILD, the fourth recess exposing the first and the second semiconductor layers in the middle portion of the semiconductor fin; after the removing the gate stack, performing an oxidation process on an outer portion of the first semiconductor layer, an inner portion of the first semiconductor layer is not oxidized; and forming a replacement gate in the recess that is in the ILD.
 11. A method of forming a Fin Field-Effect Transistor (FinFET) device, the method comprising: forming a first and a second semiconductor fin over a substrate, the first and second semiconductor fins being parallel to each other; forming Shallow Trench Isolation (STI) regions surrounding the first and the second semiconductor fins, wherein top portions of the first and the second semiconductor fins comprise a first epitaxial layer and a second epitaxial layer, with the first epitaxial layer underlying the second epitaxial layer and having a first germanium percentage; forming a gate stack over respective middle portions of the first and the second semiconductor fins; forming a fin spacer between end portions of the first and the second semiconductor fins, wherein the end portions of the first and the second semiconductor fins are disposed on a same side of the gate stack, wherein the fin spacer extends continuously on a first sidewall of the first semiconductor fin, a top surface of the STI regions, and a second sidewall of the second semiconductor fin, and wherein an edge of the fin spacer on the first sidewall has a first height larger than a second height of the fin spacer measured at a point midway between the first and the second semiconductor fins; removing top portions of the second epitaxial layer from the respective end portions of first and the second semiconductor fins; and epitaxially growing a first source/drain region and a second source/drain region over remaining portions of the second epitaxial layer of the end portions of the first and the second semiconductor fins, respectively.
 12. The method of claim 11, wherein the second epitaxial layer is substantially free of germanium.
 13. The method of claim 11, further comprising: forming a gate spacer on a sidewall of the gate stack, wherein the gate spacer and the fin spacer are formed using a same material and are connected.
 14. The method of claim 13, wherein forming the gate spacer and the forming the fin spacer comprise: forming a conformal spacer layer over the gate stack, the STI regions, and the first and the second semiconductor fins; and patterning the spacer layer to form the gate spacer and the fin spacer.
 15. The method of claim 11, wherein the removing the top portions of the second epitaxial layer forms recesses in the end portions of the first and the second semiconductor fins, wherein in the epitaxially growing the first source/drain region and the second source/drain region, the first and the second source/drain regions grow vertically first to fill the recesses, before growing laterally beyond the recesses.
 16. The method of claim 11, further comprising: forming an Inter-Layer Dielectric (ILD) over the gate stack, the first and the second semiconductor fins, and the STI regions; removing the gate stack to form a recess in the ILD, the recess exposing the first and the second epitaxial layers in the middle portions of the first and the second semiconductor fins; performing an oxidation process on exposed outer portions of the first and the second epitaxial layers without oxidizing inner portions of the first and the second epitaxial layers; and forming a replacement gate in the recess of the ILD.
 17. The method of claim 16, wherein performing the oxidization process increases a volume of the first epitaxial layer to achieve a desired degree of channel strain.
 18. The method of claim 16, wherein performing the oxidization process forms a semiconductor region in the inner portions of the first epitaxial layer, wherein the semiconductor region has a second germanium percentage higher than the first germanium percentage.
 19. A method of forming a Fin Field-Effect Transistor (FinFET) device, the method comprising: forming a first and a second semiconductor fin over a substrate, with Shallow Trench Isolation (STI) regions surrounding the first and the second semiconductor fins, wherein top portions of the first and the second semiconductor fins comprise a germanium-containing first semiconductor layer and a substantially germanium-free second semiconductor layer, with the first semiconductor layer underlying the second semiconductor layer; forming a gate stack over middle portions of the first and the second semiconductor fins; forming fin spacers on sidewalls of end portions of the first and the second semiconductor fins, wherein top ends of the fins spacers extend above a top surface of the first semiconductor layer; removing top portions of the second semiconductor layer, and subsequently, epitaxially growing source/drain regions over the end portions of the first and the second semiconductor fins; removing the gate stack to expose the first and the second semiconductor layers in the middle portions of the first and the second semiconductor fins; and oxidizing outer portions of the first semiconductor layer.
 20. The method of claim 19, wherein the oxidizing the outer portions of the first semiconductor layer forms a semiconductor oxide region in the outer portions of the first semiconductor layer, and a semiconductor region in inner portions of the first semiconductor layer, wherein the semiconductor oxide region has a first portion overlying the semiconductor region and a second portion underlying the semiconductor region. 